Token re-use API breaks back compatibility in certain TokenStream chains
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Key: LUCENE-1063
URL: https://issues.apache.org/jira/browse/LUCENE-1063
Project: Lucene - Java
Issue Type: Bug
Components: Analysis
Affects Versions: 2.3
Reporter: Michael McCandless
Assignee: Michael McCandless
Fix For: 2.3
In scrutinizing the new Token re-use API during this thread:
http://www.gossamer-threads.com/lists/lucene/java-dev/54708
I realized we now have a non-back-compatibility when mixing re-use and
non-re-use TokenStreams.
The new "reuse" next(Token) API actually allows two different aspects
of re-use:
1) "Backwards re-use": the subsequent call to next(Token) is allowed
to change all aspects of the provided Token, meaning the caller
must do all persisting of Token that it needs before calling
next(Token) again.
2) "Forwards re-use": the caller is allowed to modify the returned
Token however it wants. Eg the LowerCaseFilter is allowed to
downcase the characters in-place in the char[] termBuffer.
The forwards re-use case can break backwards compatibility now. EG:
if a TokenStream X providing only the "non-reuse" next() API is
followed by a TokenFilter Y using the "reuse" next(Token) API to pull
the tokens, then the default implementation in TokenStream.java for
next(Token) will kick in.
That default implementation just returns the provided "private copy"
Token returned by next(). But, because of 2) above, this is not
legal: if the TokenFilter Y modifies the char[] termBuffer (say), that
is actually modifying the cached copy being potentially stored by X.
I think the opposite case is handled correctly.
A simple way to fix this is to make a full copy of the Token in the
next(Token) call in TokenStream, just like we do in the next() method
in TokenStream. The downside is this is a small performance hit. However
that hit only happens at the boundary between a non-reuse and a re-use
tokenizer.
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